Photocrosslinked second order nonlinear optical polymers

ABSTRACT

A novel photocrosslinkable polymeric system has been developed for processing into films having stable second-order nonlinear optical properties. In the present system, polymers bearing photocrosslinkable chromophores, such as polyvinylcinnamate and are reacted with appropriately designed nonlinear optical molecules with the cinnamate or other photocrosslinkable functionalities for photocrosslinking at one, two or more points. The system can be poled and photocrosslinked in the poled state to yield a material with stable optical nonlinearity and large electro-optic coefficients.

BACKGROUND OF THE INVENTION

Second order nonlinear optical (NLO) polymers are expected to findextensive uses in opto-electronic applications. NLO polymers haveseveral advantages over single crystalline inorganic and organicmolecular systems. These include easy preparation, adjustable refractiveindices and controlled arrangement of spatial order. For second orderapplications it is imperative that the material be noncentrosymmetric.In noncentrosymmetric organizations several organic molecular andpolymeric systems have been characterized by large second order NLOcoefficients, ultra-fast response times, performance over a broadwavelength range and high laser damage threshold compared to thetraditional inorganic materials, e.g., lithium niobate (LiNbO₃) orpotassium dihydrogenphosphate (KH₂ PO₄). Background information relatingto the principles of nonlinear optical polymers, is contained in"Nonlinear Optical and Electroactive Polymers", edited by Prasad andUlrich, Plenum Press, (1988).

A number of applications, such as second harmonic generation (SHG),frequency mixing, electro-optic modulation, optical parametric emission,amplification and oscillation have been proposed for organic andpolymeric materials with large second order NLO coefficients. R. D.Small et al., "Molecular and Polymeric Optoelectronic Materials:Fundamentals and Applications", edited by Khanarian, SPIE, 682:160(1986). A number of approaches have been made in the past decade toorganize NLO molecules in a polymer matrix in a noncentrosymmetricmanner. The most important, but not the only aspect from the standpointof application, is the organization of NLO molecules into preferredorientation and their stability in the aligned state up to at least coldwire bond temperatures (about 100° C.).

Historically, one of the first approaches to this alignment of NLOmolecules in a polymeric system came with the concept of the"guess-host" system. Singer et al., Appl. Phys. Let., 49:248 (1986). TheNLO molecules may be incorporated by a solution casting method with anamorphous polymer and the second order nonlinearity may be imparted bysubsequent poling of the NLO molecules in the matrix using an externalelectric field, e.g., corona poling, parallel plate poling or integratedelectrode poling. Advantages of this approach are ease of processing,tailorable refractive indices, control of spatial ordering of thepolymer, and choice of a wide range of materials. However, the decay(both the initial and long term) of second order properties as confirmedthrough SHG from the matrix is unavoidable when the poling field iswithdrawn from the matrix. Moreover, a high degree of loading of the NLOmolecules in the polymer is not possible because of phase segregation ofthe matrix or blooming of NLO molecules in the matrix, both resulting inoptical scattering.

In a second approach, known as "grafted" systems, a number of newfeatures are routed just by linking NLO molecules covalently in the sidechains of a suitable polymer backbone. Meredith et al., Macromolecules,15:1385 (1982). Despite the synthetic complexity of such a system, alarge number of NLO molecules (a concentration 2 to 3 times greater thanthe guest-host system) can be coupled with the polymer side chains, yetthe polymers are easily processable. Both the initial and long termdecay in second harmonic (SH) properties are reduced to a great extent.

Recently, a three dimensional network consisting of NLO molecules, knownas the "cross-linked" system, has been developed to overcome a number ofproblems associated with the guest-host or grafted systems. Reck et al.,SPIE, 1147:74 (1989) and Eich et al., J. Appl. Phys., 66(7):3241 (1989).In this system, multifunctional epoxy and amino compounds containing NLOcomponents are simultaneously processed, poled and crosslinked tofreeze-in the nonlinear effects permanently. Properties resulting fromthe cross-linked system are significantly small decay in SH propertiesover a long period of time and the ability for processing with largeconcentrations of NLO molecules. However, for developing an optimalepoxy based NLO material precise control of the molecular weight of theprepolymer is a stringent and necessary condition. In addition, polingand curing at elevated temperatures has to be carried out over a longperiod of time (about 20 hours) making processing of the materialssignificantly difficult.

SUMMARY OF THE INVENTION

The invention relates to a novel three-dimensional polymer matrixcomprising noncentrosymmetrically aligned NLO molecules. The presentpolymers are prepared by photochemical reactions between photosensitivechromophores which have been functionalized into NLO molecules and thesame or related chromophores appended into a host polymer acting as thematrix.

The functionalized NLO molecules are at least di-functional, and formcrosslinks with the chromophores in the polymer to form the desiredthree-dimensional crosslinked network. In one embodiment, an extendedπ-conjugated diazo dye system substituted with donor-acceptor groups,and cinnamate groups, at either end are used as model NLO molecules andpolyvinylcinnamate (PVCN) is used as the model host polymer for thedemonstration of the present invention. The cinnamate groups attached tothe NLO molecules and the pendant cinnamate groups on the PVCN polymerare photocrosslinked, for example, by exposure to ultra-violet (UV)radiation, thereby forming a three-dimensional crosslinked network.

The present polymers may be cast in bulk or as films. Thin films of thepresent polymers can be produced, for example, by spin-coating from asolution containing the NLO molecules and the host polymer in an organicsolvent or mixture of organic solvents. A wide range of solvents orsolvent mixtures can be used for spin-coating.

The cast film is then poled to introduce noncentrosymmetric organizationof the NLO molecules in the polymer film. This can be accomplished byexposing the film to an electric field, for example, by corona poling.The poling temperature is usually close to the glass transitiontemperature (T_(g)) of the polymer. The process conditions are specificto the system and can be established using the appropriate routineprotocol.

The NLO molecules are then permanently frozen-in into the preferredorientation by crosslinking. Crosslinking is performed photochemically,for example, by exposure to UV irradiation. Poling and crosslinking canbe performed as independent steps, or simultaneously.

Noncentrosymmetric polymer films produced as described herein haveseveral advantages. They are easy to prepare, exhibit ultra-fastresponse times, are stable at elevated temperatures, perform over abroad wavelength range and have a high laser damage threshold. Also theprocess of poling is independent of the crosslinking process and theycan be superimposed in a desired manner.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic illustration of the photocrosslinking reactionbetween a cinnamate-functionalized conjugated NLO molecule andpolyvinylcinnamate.

FIG. 2 is a schematic illustration showing the process for synthesizingcinnamate-functionalized NLO molecules. Scheme A illustrates thesynthesis of3-cinnamoyloxy-4-[4-(N,N-diethylamino)-2-cinnamoyloxyphenylazo]nitrobenzene and Scheme B illustrates the synthesis of6-cinnamoyloxy-3-[4-(N,N-diethylamino)-2-cinnamoyloxyphenylazo]nitrobenzene.

FIG. 3 is a graph illustrating the stability of a poled PVCN filmcrosslinked with3-cinnamoyloxy-4-[4-(N,N-diethylamino)-2-cinnamoyloxyphenylazo]nitrobenzene compared to a poled and noncrosslinked PVCN film with thesame compound.

DETAILED DESCRIPTION OF THE INVENTION

The present polymers are prepared by reacting photo-reactivechromophores which are attached to NLO molecules with the same orrelated chromophores which are attached to a polymer forming the matrix.The term "NLO molecules" as used herein means molecules possessingsecond order nonlinear optical properties.

The NLO molecules are generally organic molecules possessingdonor-acceptor groups. Organic molecules which possess donor-acceptorgroups attached to an aromatic ring, which improve charge transferthrough π-electron delocalization, and which exhibit second order NLOproperties, are particularly useful as NLO molecules. Organic moleculeshaving these properties are shown schematically below as Formula 1, anda specific compound, p-nitroaniline, is shown where the donor group (D)is an amine group (NH₂) and the acceptor group (A) is a nitro group(NO₂). ##STR1## The terms "donor" and "acceptor" as used herein refer tofunctional groups which either "donate" or "accept" π electrons,respectively. The nitro group (NO₂) is the preferred π-acceptor althoughother groups such as cyano (CN) are also used. As a π-donor, nitrogen inthe form of an amine group (NH₂) is preferred, although donor groupsattached through other atoms such as oxygen, can also be employed. Forexample, donor groups which are useful in the present NLO moleculesinclude, in addition to amine (NH₂), N,N-dimethylamine (N(CH₃)₂),N,N-diethylamine (N(C₂ H₅)₂), methoxy (--OCH₃) and ethoxy (--OCH₂ CH₃).Useful acceptor groups, in addition to nitro (NO₂) and cyano (CN),include pyridinium salts and N-oxide.

The π moiety can be any system rich in π electrons, such as a bezenering. A bezene ring having substituents, such as hydroxy groups, whichdo not interfere with or which enhance the donor and acceptorcharacteristics of the ring system can also be used.

In a preferred embodiment of the present invention, conjugatedπ-extended NLO molecules are used. Efficient second order properties areobtained using NLO molecules having an extended conjugated π-electronsystem with an asymmetric charge distribution provided by donor-acceptorgroups at either end of the conjugated system. A conjugated π-extendedNLO molecule is shown schematically below as Formula 2, ##STR2## whereinX and Y represent the linking atoms to the acceptor (A) and donor (D)groups, respectively. For example, diazo, stilbene, and azomethine areuseful conjugated NLO molecules, wherein X and Y are the moieties shownin Table 1.

                  TABLE 1                                                         ______________________________________                                                    X                Y                                                ______________________________________                                        DIAZO         N                  N                                            STILBENE      CH                 CH                                           AZOMETHINE    CH                 N                                                                       or                                                               N                  CH                                           ______________________________________                                    

A method for producing conjugated NLO molecules is described in detailin Example 1. Higher magnitudes of the nonlinear effects in appropriatemolecular arrangements can be obtained by molecular engineering andsynthetic tailoring.

Any NLO molecule to which photosensitive chromophores may be attached atone, two or more points can be used in the present process. Thephotosensitive chromophores are best attached through hydroxyl groupspresent in the NLO molecule. One of the easiest ways to attachphotosensitive chromophores such as, cinnamate, to the NLO molecules isby esterification of hydroxyl groups by cinnamoyl chloride (C₆ H₅--CH═CH--COCl).

By attaching stronger electron-donating and electron-withdrawing groups,and/or increasing the length of the π-electron system, the absorptionband of the corresponding charge-transfer excitations is shifted towardslonger wavelengths. There may be an associated increase in the nonlinearoptical coefficients. Thus, the second order nonlinear response andplacement of the transparent window for optical applications can betailored by rearranging the groups in the aromatic rings of the NLOmolecules. NLO molecules selected with appropriate linear and nonlinearoptical properties can be incorporated into a stable noncentrosymmetricorganization.

The distribution of donor-acceptor groups and hydroxyl groups in the NLOmolecules has a strong effect on the second order properties of thefinal polymeric system. The NLO molecules are functionalized byattaching photosensitive chromophores to the molecule, so that they canbe photocrosslinked. In one embodiment, NLO molecules havingdiazo-linked π-extended systems were synthesized by a diazo couplingreaction of aromatic molecules substituted with donor-acceptor groupsand hydroxyl groups. Nitro groups and N,N-diethylamine groups werechosen as the donor and acceptor groups, respectively, and were placedat either end of the π-extended molecule in order to obtain large secondorder electro-optic effects. The combination is shown in two isomericstructures as Formulae 3 and 4 below wherein R is H. The hydroxyl groupswere each attached with a photosensitive chromophore, in this example acinnamate group, to create a functionalized NLO molecule, shown asFormulae 3a and 4a below, wherein R is the cinnamoyl group, C₆ H₅--CH═CH--CO. ##STR3##

In addition to cinnamate, a wide range of photosensitive chromophoresare available for making photocrosslinkable functionalized NLO moleculeswhich are useful in the present system, such as styrylacrylate (C₆ H₅--CH═CH--CH═CH--CO₂ --) or chalconeacrylate (C₆ H₅ --CO--CH═CH--C₆ H₄--CH═CH--CO₂ --). Photosensitive chromophores which are useful inphoto-crosslinking reactions are described, for example, by Reiser in:Photoreactive Polymers, Wiley & Sons, Inc., New York (1989).

In the present photocrosslinking system, a three-dimensional polymer isproduced by a photocrosslinking reaction between the photosensitivefunctional groups attached to the NLO molecules described above and thesame or similar molecules on host polymers. Thus, the structure of thepresent polymer will depend upon the choice of the functionalized NLOmolecule and the host polymer. The term "host polymer" as used hereinrefers to a polymer having photoreactive groups which are capable ofphotocrosslinking among themselves or with the photosensitive functionalgroups on the NLO molecule. Therefore, the choice of host polymer willdepend, among other things, upon the functional groups attached to theNLO molecules. For example, if an NLO molecule contains a cinnamatefunctional group, then the host polymer will be selected to have thesame or similar pendant groups which are capable of photocrosslinkingwith the cinnamate groups, such as polyvinylcinnamate (PVCN). Anypolymer having photoreactive C═C moieties can act as a host system. Forimproved photoreactivity, the double bond should have adequatepolarization and light-absorbing characteristics. For example, in PVCN,the adjacent carbonyl group provides a desirable polarization towardsthe reactivity of the double bonds. The phenyl group in PVCN, on theother hand, increases the polarizability and enhances the lightabsorbing power of the chromaphore. Polymers which are useful as hostpolymers include, in addition to PVCN, polymers containingstyrylacrylate (C₆ H₅ --CH═CH--CH═CH--CO₂ --) or chalconeacrylate (C₆H.sub. 5 CO--CH═CH--C₆ H₄ --CH═CH--CO₂ --) groups.

The reaction between a cinnamate-functionalized conjugated NLO moleculeand PVCN as the host polymer is used as a model system to illustrate thepresent invention. In this reaction, crosslinks are formed by 2+2photodimerization between an excited cinnamoyl group of the host polymerand the ground state cinnamoyl group belonging to the NLO molecule, orvice versa. The photodimerization between the two C═C double bonds ofthe cinnamoyl groups is very effective because of the adjacent carbonylgroup which provides desirable polarization towards the reactivity ofthe double bond. The phenyl group, on the other hand, increases thepolarizability and enhances the light absorbing power of thechromophore. The intermolecular photocrosslinking reaction between thephotosensitive chromophores of the polymer and the active NLO moleculesis represented schematically in FIG. 1.

Improvement of the relative photosensitivity and spectral sensitivityrange in the host polymer beyond those of PVCN is possible byintroducing a second double bond into the conjugated olefin-ketonesystem of the cinnamoyl group, as shown, for example, in Formula 5below. Certain pyridinium salts, shown as Formula 6, are also known toactivate double bonds towards photocrosslinking reactions. The spectralsensitivity range can be controlled by selection of the appropriate Rgroup. For example, in Formula 6, R can be NO₂ or OCH₃. ##STR4##

In the model system used to illustrate the present invention, the NLOmolecules are functionalized with cinnamate groups to allowintermolecular photocrosslinking reactions to occur with PVCN. Thecinnamoyl group absorbs its maximum at about 280 nanometers (nm), butthe crosslink generating photoreaction can be performed albeit, at aslower rate at a wavelength of 254 nm as well. The photocrosslinkingreaction can be enhanced, for example, by spectral sensitization by thereplacement of the cinnamate group with a chromophore that absorbs atmaximum at a longer wavelength, such as styrylacrylate, or by adding asensitizing agent.

There are a large number of compounds that sensitize PVCN in thenear-ultraviolet region and significantly increase the photosensitivityof the composition. For example, thiazolines(N-methylbenzoyl-β-naphthathiazoline), nitroaromatics(3-nitroacenaphthalene) and ketocoumarins (7-propoxy-3-benzoylcoumarin)represent three major efficient classes of sensitizers among others.They absorb at longer wavelengths and funnel this energy to a reactivesite.

An alternative sensitization approach involves the modification of thephotosensitive chromophore attached to the host polymer. Traditionally,cinnamate polymers have been prepared by the esterification ofpolyvinylalchohol, for example, by Delzenne in: Encyclopedia of PolymerScience and Technology, Suppl. Vol. 1, pp. 401 J. Wiley & Sons, Inc.,New York (1976). Copolymers of PVCN can be prepared for use as hostpolymers to address particular fabrication requirements. Thephotocross-linkable units can be made to function at 200 to 700 nm as aresult of the judicious use of specific chromophores with specifictriplet sensitization. One example of a photocrosslinkable polymer ispolyvinyl styrylacrylate (shown as above as Formula 5). This polymerabsorbs at longer wavelengths (330 nm) and has a lower triplet energythan cinnamate. Hence, the choice of the photocross-linkablepolymer/sensitizer combination permits tailoring of the system torespond to specific light sources and wavelengths.

The present three-dimensional polymers are prepared by the followinggeneral procedure. The polymers can be cast in bulk or as films. Aconventional spinning technique is employed for illustrative purposes inthe present procedure. In this process, thin films of the functionalizedNLO molecules and the photosensitive host polymers are coated oversubstrates such as glass, quartz or thermally grown silicon dioxide onsilicon. In this method, the functionalized NLO molecules and the hostpolymer are dissolved in an organic solvent or solvent mixture. Thesolution can contain broad ratios of the NLO molecule and the hostpolymer depending on the properties of the polymer which are desired.The amounts of each can be determined by routine methods. A broad rangeof solvents may also be used. The choice of solvent will depend upon thesolubility of the NLO molecules and host polymers. Polar organicsolvents are useful in this process. For PVCN, tetrahydrofuran (THF),1,4-dioxane, furfural and a toluene:1-chlorobenzene mixture (1:3), canbe used for obtaining good quality thin films.

Thin films of the polymers can be produced by spin-coating the solution.Films having a thickness of from about 0.5 μm to about 2.5 μm may beeasily obtained. Film thickness can be controlled by adjusting thespinning speed and/or the viscosity of the solution.

In another embodiment of the present method, the NLO molecules areattached to the host polymer, and a solution of the NLO-functionalizedpolymer is spin-coated onto a substrate as described above. The polymerhas pendant photoreactive chromophores, and crosslinking takes placebetween the photoreactive chromophores.

The cast films are allowed to dry, and are then "poled" to introducenoncentrosymmetric organization to the NLO molecules in the film. Thiscan be accomplished, for example by corona poling. Poling can be carriedout as described in "Electrets", Sessler (Ed.) Springer-Verland, Berlin,Germany, pp. 3 (1987). This poling temperature is usually close to theglass transition temperature (T_(g)) of the polymer. In the present PVCNsystem the poling temperature was chosen close to 70° C., which is about10° C. below the Tg of the polymer. The applied voltage on the coronawire is varied depending on the thickness of the film, concentration ofNLO molecules and the temperature selected for poling. The coronacurrent is generally in the range of from about 1.5 to about 3.5 μA.

The effective poled area, approximately 1.5 cm wide and 2.5 cm long,appears more transparent compared to the unpoled region as thechromophores are aligned normal to the film as a result of poling. Theexact process conditions will be specific to the system and can bedetermined by routine experimentation.

The NLO molecules are then permanently frozen-in into the preferredorientation by crosslinking. Crosslinking is performed photochemically,for example, by UV irradiation. The wavelength of irradiation can beselected depending on the absorption profile of the polymer matrix. Ingeneral, almost all films in which PVCN is the host polymer can bephotocrosslinked at a wavelength of 254 nm. The crosslinking reactioncan be carried out over a wide frequency range of the incident light,for example, light sources rich in the 365 nm line of mercury can beused. Or, an argon-ion laser can be used, with a small quantity of acommercially available triplet sensitizer, such as7-propoxy-3-benzoylcoumarin. Photocrosslinking can be performed at thelate stage of the poling cycle while the electric field is on, orimmediately after the poling field is removed.

The refractive index of the film can be measured by ellipsommetry. Theindex varies as a function of the concentration of NLO molecules in thematrix. Typically, a film of 1.5 μm thickness containing 20% by weightNLO molecules has a refractive index of 1.58 for the poled regions and1.63 for the unpoled regions in the plane of the film at a wavelength of532 nm.

The nonlinear optical properties of the poled films can be investigatedby using SHG as the probing technique. In this method, measurements aremade on a Q-switched Nd-YAG laser in which a polarized beam of light ispassed through the sample, and the second-harmonic coefficient of thecrosslinked polymer film, d₃₃, is obtained. Polymer matrices made by thepresent method show no decay in SHG over long periods of time atelevated temperatures, for example, ranging from about 60° to 85° C.

In the model system, for example, a Q-switched Nd:YAG laser (λ=1064 nm)with a pulse width of 10 nanoseconds (ns) and a pulse energy of 45millijoules (mJ) was used as the fundamental source, and a referencesample of Y-cut quartz (d₁₁ =0.364 pm/V) was used for the calibration ofthe frequency doubled signal. The second harmonic coefficient of thepolymer film, d₃₃, of the model system, was obtained from a Maker fringeanalysis of the data. Jeophagnon et al., J. App. Phys., 41:1667 (1970).The value of d₃₃ varied from 15 to 30 picometers/volt (pm/V) dependingon the concentration of NLO molecules. By optimizing the degree ofalignment and the concentration of the NLO molecules in the polymermatrix, the second order coefficient can be further varied.

The three-dimensional, nonlinear optical polymers of the presentinvention have large second order NLO properties. The present polymersexhibit many desirable processing characteristics, such as excellentnegative resist and poling and doping features. The present polymers arepoled, and photocrosslinked in the poled state to yield a materialhaving excellent optical quality, stable optical nonlinearity and largeelectro-optic coefficients.

The present NLO polymers can be used in a number of applications, suchas second harmonic generation (SHG), frequency mixing, electro-opticmodulation, optical parametric emission, amplification and oscillation.The use of polymerc materials having large second order NLOco-efficients, such as the present polymers, is described by R. D. Smallet al., in: "Molecular and Polymeric Optoelectronic Materials:Fundamentals and Applications", edited by Khanarian, SPIE, 682:160(1986).

The invention will now be illustrated by the following examples.

EXAMPLES EXAMPLE 1

The synthetic route for obtaining photosensitive chromophore-substituteddiazo dyes is shown schematically in FIG. 2. The following reaction isshown in FIG. 2A. In this reaction, 18.63 g of 2-amino-5-nitrophenol wasdissolved in 75 ml of concentrated sulfuric acid and 75 ml of water.8.34 g of sodium nitrite dissolved in 100 ml of water was slowly addedat temperatures below 5° C. To the cooled solution, 115 ml of 2N sodiumhydroxide solution with 19.98 g of 3-diethylaminophenol was added andthe reaction mixture was stirred overnight at room temperature. Theproduct, 3-hydroxy-4-[4-(N,N-diethylamino)-2-hydroxyphenylazo]nitrobenzene, shown above as Formula 3 (wherein R=H), was filtered,washed with water and dried under vacuum at 60° C. The dye was purifiedby crystallization from chloroform.

Cinnamoyl chloride (3.15 g) in 10 ml THF was added dropwise to asolution of 1 (3.12 g) and triethylamine (3 g) in THF (10 ml) andstirred for 12 h at room temperature. The solvent was removed underreduced pressure, the residue was dissolved in dichloromethane, washedwith water and dried over magnesium sulfate (MgSO₄). The product,3-cinnamoyloxy-4-[4-(N,N-diethylamino)-2-cinnamoyloxyphenylazo]nitrobenzene, shown above as Formula 3a, was purified by columnchromatography (silica gel, THF as eluent) as a deep red solid.

1 g of PVCN and 0.2 g of 1a were dissolved in 5 ml of 1,4-dioxane usingan ultrasonic mixer at 35° C. The resulting solution was used tospin-coat a glass substrate at 1250 to 4000 rpm for 1 min. Prebaking ofthe sample was done immediately after spin-coating at 60° C. for 12hours.

The substrate was kept on the hot-stage of the poling equipment for 1minute at 60° C. prior to the poling, and photocrosslinking cycles. Thefilm was corona poled for 3 min. The applied voltage on the corona wirewas maintained at 6 kV while the corona current of approximately 2 μAwas established. The poled film was then cross-linked for 10 minutes byUV irradiation. A radiation dosage of 2.5 to 3 mW/cm² at wavelength 254mn was maintained during the crosslinking reaction. Sudden cooling ofthe substrate was done after UV irradiation by passing cold waterthrough the hot-stage. The poling field was kept on during both theradiation and cooling cycles.

The results are shown in Table 2 below, and in FIG. 3. As shown in FIG.3, the poled PVCN film crosslinked with compound 3a is much more stablethan a poled, uncrosslinked PVCN film.

                  TABLE 2                                                         ______________________________________                                        Optical properties of PVCN film doped with 3a                                            PVCN/3a (10%)                                                                            PVCN/3a (20%)                                           ______________________________________                                        Thickness (μm)                                                                          0.5          0.5                                                 Abs. max (nm)                                                                              520          520                                                 T.sub.g (°C.).sup.a                                                                 84           81                                                  Refractive index                                                              λ (μm)                                                              0.532        1.632        1.634                                               0.632        1.677        1.685                                               1.000        1.613        1.625                                               d.sub.33 (pm/V).sup.b                                                                      11           16.5                                                ______________________________________                                         .sup.a Obtained from DSC (DuPont 2910 differential scanning calorimeter),     10° C./min (midpoint). T.sub.g of PVCN is 88° C.                .sup.b d.sub.33 not corrected for absorption.                            

EXAMPLE 2

The following reaction is shown schematically in FIG. 2B. In thisreaction, 15.41 g of 4-amino-2-nitrophenol was dissolved in 60 ml ofconcentrated sulfuric acid and 60 ml of water. 6.9 g of sodium nitritedissolved in 100 ml of water was slowly added at temperatures below 5°C. To the cooled solution, 100 ml of 2N sodium hydroxide solution with16.52 g of 3-diethylaminophenol was added and the reaction mixture wasstirred overnight at room temperature. The product,6-hydroxy-3-[4-(N,N-diethylamino)-2-hydroxyphenylazo] nitrobenzene,shown as Formula 4 above (wherein R=H), was filtered, washed with waterand dried under vacuum at 60° C. The dye was used purified by columnchromatography over silica gel (using dichloromethane/diethylether[1:1]as eluent) as a pink crystal.

Cinnamoyl chloride (1.02 g) in 5 ml THF was added dropwise to a solutionof dye 2 (1 g) and triethylamine (0.77 g) in THF (10 ml) and stirred for12 h at room temperature. The solvent was removed under reducedpressure, the residue was dissolved in dichloromethane, washed withwater and dried (MgSO₄). The product,6-cinnamoyloxy-3-[4-(N,N-diethylamino)-2-cinnamoyloxyphenylazo]nitrobenzene, shown as Formula 4a above, was purified by columnchromatography (silica gel, THF as eluent) as an orange solid.

1 g of PVCN and 0.2 g of 4a were dissolved in 5 ml of 1,4-dioxane usingan ultrasonic mixer at 35° C. The resulting solution was used tospin-coat a glass substrate at 1250 to 4000 rpm for 1 min. Prebaking ofthe sample was done immediately after spin-coating at 60° C. for 12hours.

The substrate was kept on the hot-stage of the poling equipment for 1minute at 60° C. prior to poling and photocross-linking cycles whichwere 3 min and 10 min, respectively, as described in Example 1. Theapplied voltage on the corona wire was maintained at 6 kV while thecorona current of approximately 2 μA was established. A radiation dosageof 2.5 to 3 mW/cm² at wavelength 254 nm was maintained during thecross-linking reaction. Sudden cooling of the substrate was done afterUV irradiation by passing cold water through the hot-stage. The polingfield was kept on, during both the radiation and cooling cycles. Thecrosslinked polymer film was much more stable than a noncrosslinkedsample of the film.

Equivalents

Those skilled in the art will recognize, or be able to ascertain usingno more than routine experimentation, many equivalents to the specificembodiments of the invention described herein. Such are intended to beencompassed by the following claims.

We claim:
 1. A photocrosslinkable nonlinear optical polymer composition,comprising:a) a nonlinear optical component which includes at least twophotosensitive functional groups; and b) a host polymer component whichincludes a plurality of photosensitive functional groups, whereby,during exposure of the photocrosslinkable nonlinear optical polymercomposition to sufficient electromagnetic radiation, at least two of thephotosensitive functional groups of the host polymer component reactwith at least two of the photosensitive functional groups of thenonlinear optical component to cause the nonlinear optical component tocrosslink with the host polymer component, thereby forming aphotocrosslinked nonlinear optical polymer.
 2. A composition of claim 1wherein the nonlinear optical component includes a nonlinear opticalcompound having an organic moiety, an electron-donating group bonded tothe organic moiety, and an electron-accepting group bonded to theorganic moiety.
 3. A composition of claim 2 wherein the organic moietyincludes an aromatic ring structure.
 4. A composition of claim 2 whereinthe electron-donating group of the nonlinear optical molecule isselected from the group consisting of amine, ethylamine, methoxy andethoxy functional groups.
 5. A composition of claim 4 wherein theelectron-accepting group of the nonlinear optical molecule is selectedfrom the group consisting of nitro, nitroso, cyano and pyridinium saltfunctional groups.
 6. A composition of claim 5 wherein thephotosensitive functional groups of the nonlinear optical component areselected from the group consisting of cinnamates, styrylacrylates andchalconeacrylates.
 7. A composition of claim 6 wherein the nonlinearoptical molecule includes an extended π-conjugated moiety.
 8. Acomposition of claim 7 wherein the extended π-conjugated moiety isselected from the group consisting of stilbene dyes, azomethine dyes anddiazo dyes.
 9. A composition of claim 8 wherein the photosensitivefunctional groups include a photoreactive carbon-carbon double bond. 10.A composition of claim 9 wherein the host polymer component includes ahomopolymer.
 11. A composition of claim 10 wherein the host polymercomponent is selected from the group consisting of polyvinylcinnamate,polyvinylstyrylacrylate and polyvinylchalcone.
 12. A composition ofclaim 11 further comprising a spectral sensitizing agent.
 13. Acomposition of claim 12 wherein the spectral sensitizing agent isselected from the group consisting of thiazolines, nitroaromatics andketocoumarins.
 14. A composition of claim 9 wherein the host polymercomponent includes a copolymer.